Abstract
The understanding of wakes of wind energy converters (WEC) is highly relevant to wind-energy research. In orographically complex terrain, wind is amplified at ridges but also varies significantly, exhibiting strong gustiness and enhanced turbulence levels compared to wind conditions in flat terrain. In this study, long-range lidar instruments are used to detect and analyse the wake of a single WEC in complex terrain in different atmospheric stability regimes. The special orography of two parallel mountain ridges at the Perdigão 2017 experiment allowed to place two lidars in a coplanar set-up and retrieve horizontal as well as vertical wind speed in a cross-section of the terrain which is in-plane with the WEC. In cases of main wind direction, which is parallel to the lidar scans, the wake’s propagation can thus be measured far downstream in the valley. A wake tracking algorithm is proposed to automatically detect the wake center in the lidar scans for three periods with distinct atmospheric stability conditions. Wind speed deficits and wake propagation paths are quantified and categorized accordingly. A careful uncertainty estimation is done for the coplanar wind retrieval.
Highlights
Wakes of wind energy converters (WEC) are highly dynamic, local features in the atmospheric boundary layer (ABL)
While the wake is following the terrain into the valley in stable stratification, it is lifted in the convective case and so in the near-neutral stratification
The fact that the first detected wake heights for convective and near-neutral case are at 30 m above hub height in the average is due to the strong vertical component of the flow being accelerated over the ridge, whereas in the stable case the flow follows the terrain
Summary
Wakes of wind energy converters (WEC) are highly dynamic, local features in the atmospheric boundary layer (ABL). The understanding of their interaction with the turbulent background wind field is challenging, especially in complex terrain. The most promising tool to gain better understanding of the physics of wakes at the present state-of-the-art are numerical large-eddy simulations (LES) [3, 4, 5, 6, 7]. These simulations can reproduce realistic atmospheric flows and wind turbine wakes, but need validation by real-world experiments.
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